The invention relates to couplings in general, and more particularly to improvements of couplings which can be resorted to for transmission of torque from a rotary driving member to a rotary driven member which should be detachable from the driving member. Couplings of such character are disclosed in numerous patents of the assignee of the present application including U.S. Pat. Nos. 4,344,304, 4,362,412, 4,406,029, 4,460,134, 4,551,117, 4,681,478 and 4,758,113. One presently preferred use of couplings of the type to which the present invention pertains is to transmit torque between a motor-driven shaft and the mandrel for a reel in a machine for winding or unwinding webs of paper, textile material, metallic foil, plastic foil or the like.
The aforementioned U.S. Pat. No. 4,551,117 discloses a coupling wherein a wheel which is movably mounted on the end portion of a motor-driven shaft must be pivoted from an operative to an inoperative position in order to permit withdrawal of the end portion of a reel-supporting mandrel from its socket in the wheel. The wheel is pivotable about an axis which extends at right angles to and is spaced apart from the axis of the motor-driven shaft. Reference may also be had to German Pat. No. 917,592 which discloses that the socket in the pivotable wheel can be surrounded by a polygonal surface having a square outline which is complementary to the outline of the polygonal external surface of the end portion of the driven member. This ensures that the coupling of the German patent is ready to transmit torque from the driving member to the driven member as soon as the wheel reassumes its operative position. A drawback of such proposal is that the socket cannot receive the end portion of the driven member with a minimum of clearance so that the end portion is likely to wobble in its socket, especially if the driven member is to be rotated at a high speed or at a very high speed. Moreover, heretofore known couplings cannot prevent excessive bending of the driven member, especially at elevated speeds, not only because of excessive play between the end portion of the driven member and the surface surrounding the socket in the pivotable wheel on the end portion of the driving member but also because the end portion of the driven member is relatively short. This is due to the fact that the provision of a socket in the pivotable wheel imposes limits upon the depth of the socket (as measured in the axial direction of the driven member). The situation is aggravated as the wear upon the parts of the coupling increases after extended periods of use and repeated insertion of the end portion of the driven member into and withdrawal of such end portion from the socket in the wheel.
The aforementioned commonly owned U.S. Pat. No. 4,362,412 proposes to overcome the drawbacks of the coupling which is disclosed in German patent No. 917,592 by the provision of an insert which defines the socket and is made of a highly wear-resistant material. In addition, the insert is removably installed in the coupling so that it can be replaced after a certain period of use. Such proposal constitutes a substantial improvement insofar as the useful life of the coupling is concerned. However, the axial length of the socket for the end portion of the driven member is still relatively short in order to permit the wheel to pivot between the ,operative and inoperative positions. This imposes limits upon the magnitude of bending and/or other stresses which can be successfully resisted by the coupling, particularly by the end portion of the driven member. Excessive stressing of the driven member entails pronounced bending which causes the driven member to run out of true and to bring about excessive wear upon the end portion of the driven member as well as on other parts of the coupling. Therefore, even a renewed and rather frequent replacement of the aforementioned insert with a fresh insert cannot prevent premature destruction of the entire coupling. The wear upon the coupling which is disclosed in U.S. Pat. No. 4,362,412 is especially pronounced when the driven member must be rotated at a very high speed such as is expected in a modern web winding or unwinding machine. The only acceptable alternative is to reduce the RPM of the driving member and to thus limit the output of the machine.